1. Field of the Invention
This invention relates to optical input scanners for providing electronic signals descriptive of information in a document.
2. Description of the Prior Art
In the conventional optical input scanner that uses a rotatable scanning mirror, a flat original is scanned by a moving light beam. A slit portion of the original is imaged on a projection image plane (such as a photoelectric sensor, or photosensor) by a projection optical system. However, the length of the light path between the original surface and the scanning mirror varies with the rotation of the mirror. Therefore, a problem arises in that the projected image does not remain in focus during the scan. Accurate conversion of the image to an electronic signal is thereby impeded.
In U.S. Pat. No. 2,262,584, issued to Herriott, a system is described for scanning a picture, printed page or other copy to derive image signals to be transmitted to a distant point. An oscillating mirror sweeps a spot of light from an illuminated aperture and a lens system provides a conjugate focal relation between the aperture and the copy. A reciprocating movement of an element of the lens system, synchronized with the mirror oscillations, is described as maintaining the conjugate focal relation independent of changes in the angular position of the beam and consequent changes in total light path length.
In U.S. Pat. No. 4,213,690, issued to Sugiura et al., a copying apparatus is described which includes a light path length correcting lens system. At least two movable lens groups are said to correct the length of the light path while maintaining a predetermined image forming magnification.
In U.S. Pat. No. 3,537,373, issued to Land, two lens elements within a projection optical system are described as being synchronously operated in parallel to the optic axis. Variation in the length of the light path and fluctuation of the conjugate arrangement is said to be eliminated.
U.S. Pat. No. 4,314,154, issued to Minoura et al, U.S. Pat. No. 4,274,733, issued to Sugiura et al., and U.S. Pat. No. 4,259,004, issued to Tateoka et al, are representative of further disclosures wherein apparatus is disclosed as comprising a scanning lens, imaging optical system, or image forming optical system that is adapted for focusing a light beam from a movable mirror onto a photosensitive member. Such apparatus are said to be capable of compensating the change in the light path length due to the angular sweep of the scanning beam.
Prior art approaches to the problem typically utilize moveable multiple-element optical devices. The elements include complex and costly lenses, which must be moved over relatively long distances along the optical axis of the apparatus. The lenses are moved in alignment with the optical axis. Such optical devices in the prior art are therefore complex in their operation and expensive to design and fabricate.